Whiteout (weather)

From Wikipedia, the free encyclopedia

Conditions approaching a whiteout in Minnesota, on March 1st, 2007. Note the unclear horizon near the center.

Whiteout is a weather condition in which visibility and contrast are severely reduced by snow and diffuse lighting from overcast clouds.

There are three different forms of a whiteout:

In blizzard conditions, the windblown snow in the air may simply make it too difficult to see very far.
In snowfall conditions, the volume of snow falling may simply obscure objects reducing visibility.
In clear air conditions, when there is no snow falling, diffuse lighting from overcast cloud may cause all surface definition to disappear. It becomes impossible to tell how far away the snowy surface is. In polar regions (and in eg the Scottish Hills in winter) this optical illusion can make whole snow-covered mountains invisible against the background white cloud, and the horizon cannot be identified, slopes cannot be judged for steepness, and snow surfaces cannot be seen. (This effect is accentuated by a smooth surface of fresh-fallen snow). It is only when a dark or coloured object is placed on a snowy surface that the surface can be seen at all. (In less extreme cases, it may suffice to break the snow surface by gently throwing a snowball ahead).

Whiteout conditions pose the greatest threat to aviation and mobile ground traffic operations. Motorists, especially those on large high speed routes are also at serious risk. There have been many major multiple-vehicle collisions associated with whiteout conditions.